This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface. Therefore newer sediment is continually deposited on top of previously deposited or older sediment. In other words, as sediment fills a depositional basins we would expect the upper most surface of the sediment to be parallel to the horizon.
Subsequent layers would follow the same pattern. As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type. For purposes of relative dating this principle is used to identify faults and erosional features within the rock record. The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across.
Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses. Once a rock is lithified no other material can be incorporated within its internal structure. In order for any material to be included within in the rock it must have been present at the time the rock was lithified. For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface. Therefore, the piece, or inclusion, must be older than the material it is included in.
Lastly the Principle of Fossil Succession. Because science advances as the technology of its tools advances, the discovery of radioactivity in the late s provided a new scientific tool by which actual ages in years can be assigned to mineral grains within a rock. This was how scientists of that time interpreted Earth history, until the end of the 19th Century, when radioactivity was discovered.
This discovery introduced a new dating technology that allows scientists to determine specific numeric ages of some rocks, called absolute dating. The next sections discuss this absolute dating system called radio-isotopic dating. All elements on the Periodic Table of Elements see Chapter 3 contain isotopes. An isotope is an atom of an element with a different number of neutrons. For example, hydrogen H always has 1 proton in its nucleus the atomic number , but the number of neutrons can vary among the isotopes 0, 1, 2.
Recall that the number of neutrons added to the atomic number gives the atomic mass. When hydrogen has 1 proton and 0 neutrons it is sometimes called protium 1 H , when hydrogen has 1 proton and 1 neutron it is called deuterium 2 H , and when hydrogen has 1 proton and 2 neutrons it is called tritium 2 H. Note that the atomic mass of elements on the Periodic Table is usually expressed with decimal digits. This indicates that the atomic mass of that element in nature is made of all its natural isotopes so the average atomic mass including all these isotopes is a decimal value.
Many elements like hydrogen have both stable and unstable isotopes.
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- 7 Geologic Time – An Introduction to Geology!
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Unstable isotopes called radioactive isotopes spontaneously decay over time releasing radiation. When this occurs, that isotope becomes an isotope of another element. This process of radioactivity is called radioactive decay. On the left, 4 simulations with only a few atoms. On the right, 4 simulations with many atoms. The radioactive decay of any individual atom is a completely unpredictable and random event. However, given a large number of radioactive atoms any measurable quantity of a substance contains trillions of atoms , the decay of half of the atoms in the specimen takes a specific amount of time.
This amount of time for half the atoms to decay is called the half-life. In other words, the half-life of an isotope is the amount of time it take for half of an initial quantity of unstable isotopes to decay into another isotope. The half-life is constant for a given radioactive isotope and can be measured. The known half-life of an isotope can be used to calculate the age of a rock. The principles behind this dating method require two key assumptions.
First, the mineral grains containing the isotope form at the same time as the rock, such as a mineral in an igneous rock that crystallized from magma. Second, the mineral crystals remain a closed system , that is they are not subsequently altered by elements moving in or out of them. These requirements place some constraints on the kinds of rock that are suitable for dating, igneous rock being the best.
Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization. Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated minerals such as evaporites may contain elements suitable for radio-isotopic dating. Igneous pyroclastic layers and lava Liquid rock on the surface of the Earth.
Cross-cutting igneous rocks and sill A type of dike that is parallel to bedding planes within the bedrock. Knowing that the zircons in the metamorphosed sediments came from older rocks, their ages established the age of the source rocks , which are no longer available for study. Two protons and two neutrons leave the nucleus. When an atom decays by alpha decay , an alpha particle is emitted from its nucleus as an alpha ray. The alpha particle consists of two protons and two neutrons, a total of four particles. This happens also to be the nucleus of a helium atom; helium gas may thus be trapped in the crystal lattice of a mineral in which alpha decay has taken place.
The loss of two protons from the nucleus of the atom lowers its atomic number by two forming an atom of an element two atomic numbers lower on the Periodic Table of the Elements. The half-life of U is 4. This isotope of uranium U can be used to determine the age of the oldest materials found on Earth, even meteorites and materials from the earliest events in our solar system.
When an atom decays by beta decay , a neutron in its nucleus splits into an electron and a proton. The electron is emitted from the nucleus as a beta ray. The new proton increases the atomic number by one and a new element is formed, but the atomic mass does not change. The process of decay of radioactive elements like uranium leads to a series of parents and daughters , each one radioactive , until a stable non- radioactive daughter is formed.
Such a series is called a decay chain. Uranium decays through a series of alpha red arrows and beta decays blue arrows to form the stable daughter product lead Pb.
7 Geologic Time
The two paths of electron capture Electron capture: In this type of radioactivity , a proton in the nucleus captures an electron from one of the electron shells to become a neutron. The result leaves two different effects: The atomic number is reduced by one and the mass number remains the same. An example of an element that decays by electron capture is potassium 40 K. Natural potassium is mostly not radioactive , but a tiny percent 0.
Both argon and calcium can be chemically separated but since calcium is very common in nature, potassium-argon is the pair that is used in dating. The half-life of 40 K in its decay to 40 Ar is 1.
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Below is a table of some of the more commonly-used radioactive dating isotopes. Some common isotopes used for radio-isotopic dating. Given a sample of rock, how is the dating procedure carried out? Using chemical analysis, the parent elements and daughter products can be separated out of the mineral. Remember that elements behave chemically due to their atomic number. As these isotopic beams pass through the instrument, the path of the heavier isotope is deflected less so the two beams strike a sensor at different places. From the intensity of each beam, the amount of parent and daughter products are determined, and from this ratio the age can be calculated.
Graph of number of half lives vs. The ratio is then 1. This can be further calculated for a series of half lives as shown in the table below. Note that after about ten half lives, the amount of parent remaining is so small that accurate chemical analysis of the parent is difficult and the accuracy of the method is diminished.
Ten half lives is generally considered the upper limit for use of an isotope for radio-isotopic dating. Modern applications of this method have achieved remarkable accuracies of plus or minus two million years in 2. The existence of these two clocks in the same sample gives a cross check on each other. Ratio of parent to daughter in terms of half-life. Schematic of carbon going through a mass spectrometer. Another radio-isotopic dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone.
Carbon dating uses the unstable isotope carbon 14 C and the stable isotope carbon 12 C. Carbon is constantly being created in the atmosphere by the interaction of cosmic particals with atmospheric nitrogen 14 N. The cosmic particles include neutrons that strike the nitrogen nucleus kicking out a proton but leaving the neutron in the nucleus. The atomic number is reduced by one from 7 to 6 forming carbon and the mass number remains the same at The 14 C quickly bond Two or more atoms or ions that are connected chemically.
However, when it dies, the radiocarbon clock starts ticking as the 14 C decays back to 14 N by beta decay with a half-life of 5, years. The radiocarbon dating technique is thus useful for about ten half lives back 57, years or so. Since radio-isotopic dating relies on parent and daughter ratios and the amount of parent 14 C needs to be known, early applications of 14 C dating assumed the production and concentration of 14 C in the atmosphere for the last 50, years or so was the same as today.
But production of CO 2 since the Industrial Revolution by combustion of fossil fuels in which 14 C long ago decayed has diluted 14 C in the atmosphere leading to potential errors in this assumption.
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Other factors affecting the estimates of composition of parent carbon in the atmosphere have also been studied. Comparisons of carbon ages with tree ring data and other data for known events have allowed calibration for reliability of the radiocarbon method which is primarily used in archaeology and very recent geologic events. Taking into account these factors, carbon dating has been shown to be a reliable dating method in this range. After the Renaissance and with the work of Hutton and others gaining attention, the idea of an ancient Earth began to be explored. Lord Kelvin applied his knowledge of physics and the assumption that the Earth started as a hot molten sphere to estimate that the Earth is 98 million years old, but because of uncertainties in his calculations, he stated it as between 20 and million years.
This estimate of an old Earth was considered plausible but not without challenge, and the discovery of radioactivity provided a better method for determining ancient ages. Patterson analyzed meteorite samples for uranium and lead using a mass spectrometer. The current estimate for the age of the Earth is 4. It is remarkable that Patterson, a graduate student in the s, came up with a result that has been little altered in over 60 years even as technology has improved the methods.
Radioactive isotopes of elements that are common in mineral crystals are useful for radio-isotopic dating. Some amazing work on zircon grains has been done. The zircon grains were incorporated in younger host rocks metasedimentary that were not that old, but the zircon grains themselves were dated at 4.
From other properties of the zircon crystals, these researchers concluded that not only were continental rocks present, but that conditions on the early Earth were cool enough for liquid water to exist on the surface and for processes of weathering and erosion to take place. Researchers at UCLA studied 4. These studies illustrate that both science and its conclusions advance as technology-driven advancements in scientific tools and ideas generate new knowledge. The rocks best suited for radio-isotopic dating are igneous , which provide dates on crystallization of primary minerals from magma.
Metamorphic processes tend to reset the clocks and smear the dates over the metamorphic events. Detrital sedimentary rocks are made of minerals derived from multiple parent sources with potentially many dates.
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However, there are igneous events that do allow dating of sedimentary sequences. For example, a lava Liquid rock on the surface of the Earth. A layer of volcanic ash Volcanic tephra that is less than 2 mm in diameter. Tephra type can be used as an adjective, i. If deposited hot, where material can fuse together while hot, the rock is then called a welded tuff.
A sill A type of dike that is parallel to bedding planes within the bedrock. Use of primary sedimentary minerals , with radioactive isotopes like 40 K, has provided dates for important geologic events. Thermoluminescence, a type of luminescence dating Luminescence: Radio-isotopic dating is not the only way scientists determine numeric ages. Luminescence dating stimulates the release of electrons that are trapped in mineral grains as radioactive isotopes decay over time. The accumulation of electrons is governed by the rate of background radiation.
The electrons are released when exposed to heat or light depending on the technique. This technique shows the last time mineral grains in a sediment or rock were exposed to light or heat. Luminescence dating is generally only useful for dating sediments that are less than 1 million years old. Fission track dating relies on damage to the crystal lattice produced when the unstable U decays to the daughter product Th releasing an alpha particle.
These two decay products move in opposite directions from each other through the crystal lattice leaving a visible track of damage.
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The tracks are large and can be visually counted under an optical microscope. The number of tracks correspond to the age of the grains. Fission track dating works from about 0. Fission track dating has also been used as a second clock to confirm dates obtained by other methods. Fossils are any evidence of past life preserved in the rocks. They may be actual remains of body parts rare , impressions of soft body parts, cast Material filling in a cavity left by a organism that has dissolved away. Life today has body parts ranging from hard bones and shells to the soft cellulose of plants, to soft-bodied organisms like jellyfish, down to single celled bacteria and algae.
Which body parts can be preserved? Thus, even in the ocean, the likelihood of preservation is quite limited. For terrestrial life, possible burial and preservation of remains is even more limited. The fossil record is incomplete, and records only a small percentage of life that existed. Although incomplete, fossils are used for stratigraphic correlation the Principle of Faunal Succession and provide a method used for establishing the age of a formation on the Geologic Time Scale. Trilobites had a hard exoskeleton. As an early arthropod, it is in the same group that includes modern insects, crustaceans, and arachnids.
Remnants or impressions of hard parts such as a marine clam shell or dinosaur bone are the most common types of fossil. The original material of these hard parts has almost always been replaced with new minerals. These minerals preserve much of the shape but the original material is gone. The following are types of fossil preservation. Actual preservation is a rare form of fossilization where actual materials of the organism or hard parts are preserved. Mosquito preserved in amber This can be unaltered preservation in amber, or preservation of original minerals like mother-of-pearl on the interior of a shell.
Another is the preservation of mammoth skin and hair in post- glacial deposits in the Arctic regions. Preservation of soft-tissue is very rare since these organic materials can easily disappear by bacterial decay. Body structures can be preserved in great detail, but stronger materials like bone and teeth are the most likely to be preserved. Petrified wood is an example where details of cellulose structures in the wood are preserved.
If the mold Organic material making a preserved impression in a rock. Sometimes internal cast Material filling in a cavity left by a organism that has dissolved away. Such internal cast Material filling in a cavity left by a organism that has dissolved away. External mold of a clam details of soft structures. If the chemistry is right, and burial is rapid, mineral nodules may form around soft structures preserving three-dimensional detail. This is called authigenic mineralization. Examples are leaf and fern fossils.
Trace fossils are indirect evidence of life left behind as it lived its life, such as burrows and tracks. Ichnology is the study of prehistoric animal tracks. Foot prints of the early crocodile Chirotherium Dinosaur tracks testify of their presence and movement over an area, and even provide information about their size, gait, speed, and behavior. Burrows dug by tunneling organisms tell of their presence and mode of life. These provide information about diet and lifestyle of the organism. That ancient life forms evolved to produce the variety of fossils we see in the rocks is important to stratigraphic correlation.
Here is a brief discussion of evolution to provide some understanding of the process. Darwin recognized that life forms evolve into progeny life forms. The mechanism he proposed for this process was Natural Selection operating on species that live within environmental conditions that pose challenges to survival.
The basic unit of classification of life on Earth is the species , a population of organisms within which individuals can mutually reproduce to produce fertile offspring. Within that population exists variations, differences in physical and behavioral characteristics. Just think of all the different variations that exist among human beings in a classroom or community. But each individual is faced with the challenges to survival posed by the environment and must survive to reproduce within those challenges.
If within the variations present in the population there are individuals that possess characteristics giving them some advantage in facing the environmental challenges, those individuals will be favored in reproducing and those favored characteristics will be passed on in successive generations.
Sufficient such favored changes in characteristics over time may cause reproducing populations to become geographically or even genetically isolated from one another eventually resulting in separate species. Evolution is well beyond the hypothesis stage and is a well-established Theory of modern science. Variation within populations occurs by natural mixing of the genes through reproduction, and also by mutations which are spontaneous changes within the genetic material DNA caused by many natural agents and processes.
Most mutations are not advantageous and soon disappear, but some cause a dramatic change in the characteristics even introduction of something novel that may be advantageous. While fossils of some species in the fossil record show little morphological change over time, others show gradual or punctuated changes within which all intermediate forms can be seen. The average lifespan of a species in the fossil record is around a million years.
That life still exists on Earth shows the role and importance of evolution as a natural process in meeting the continual challenges posed by our dynamic earth. Image showing fossils that connect the continents of Gondwana the southern continents of Pangea. Wegener used correlation to help develop the idea of continental drift.
At any given time on Earth, preservable fossils represent a sample assemblage of organisms living at that time. While the ranges in geologic time of individual fossil species vary, the assemblage of fossils is unique to the time in which it lived. Assemblages of fossils thus can be used to identify rocks of similar age at geographically dispersed locations on Earth and for assigning rocks to the systems of the Geologic Time Scale.
The process of relating rocks to each other is called correlation.